1. Field of the Invention
This invention relates to a four wheel drive working vehicle having transmission clutches separately operable for driving right and left rear wheels.
2. Description of the Related Art
As one example of the four wheel drive working vehicle noted above, there is a riding type rice planting machine disclosed in Japanese Patent Publication Kokai No. 1-293234, for example. In this working vehicle, power outputted from a transmission is divided by a rear differential for separately driving right and left rear wheels through a pair of transmission clutches, usually called side clutches, corresponding to the respective rear wheels. This working vehicle further includes side brakes for acting on the respective rear wheels. Power outputted from the transmission for driving front wheels is also divided by a front differential for transmission to the right and left front wheels.
The rear differential is switchable between a locked state and an unlocked state to produce two turning modes suited to the two states. That is, when one of side brake pedals is depressed with the rear differential locked, the transmission clutch connected to the rear wheel braked by the corresponding side brake is disengaged. Consequently, the vehicle may be turned with one rear wheel stopped even if the rear differential is locked. When the rear differential is unlocked, interlocking between the side brake pedal and side clutch is broken. In this condition, the transmission clutch connected to the rear wheel braked by the corresponding side brake remains engaged, thereby allowing the vehicle to make a turn with power distributed to the right and left rear wheels. Thus, when the vehicle is driven straight or slightly turned on a normal working site, the rear differential is maintained in operative state. If the rear differential is locked when the vehicle makes a turn with a small radius of turning, the transmission clutch connected to the rear wheel braked by one of the side brakes and lying on the inside of the turn is disengaged. Although running performance of the vehicle is thereby improved, a complicated control structure is required for locking the rear differential and operating the clutches. This results in enlargement and high cost of the apparatus relating to such controls.